What is a symmetry? It's some kind of change or process that leaves a system looking the same as when you started. Essentially, it's "a change that's not a change". If that sounds like a paradox, it's because it is.
The classic example of symmetry is rotations of a shape: if you rotate a square by 90° it will look the same as when you started — as long as the different parts of the square have no other distinguishing features (such as the numbers in the picture below). It won't be deformed or broken, and will occupy the exact same location in space.
|image from http://mathonline.wikidot.com/the-group-of-symmetries-of-the-square|
But the right side of the square cannot literally be the exact same as the left side, otherwise it wouldn't have four sides, it would have at most three. If the parts of the square weren't separate and distinct we couldn't even recognize a process as different from doing nothing — except maybe by looking at intermediate states of the rotation, to see that yes, the square is moving from one place to another. In other words, the parts have distinct identities. But in reality identities are only distinguished by observable properties — by Leibniz's identity of indiscernibles, two different things must differ on some property.
The same is true of Model A: EIIs and SLEs are "the same" by virtue of being socionic types, which have strengths, weaknesses, values, etc. described in terms of the same functions and IM elements, but they clearly don't have the same thinking and behavior. If all the types were the same then socionics wouldn't be very interesting. In fact they wouldn't even be types, as everyone would be the same.
Of course socionics acknowledges these differences already — in type descriptions the differences are described, albeit often not very well, or using overly-specific examples drawn from the author's own limited and potentially flawed observations of the type. The real issue is that the differences are not explicit in the theory. There is a mathematical structure describing precisely how the relationships and functions fit together, but there is no mathematical description of, e.g. what makes Te different from Ti. This means that anyone and everyone can come up with their own definitions and say that they're right. Certain ones may fit together better and describe reality better, but good luck trying to convince anyone else to change their minds. So we have a community which is becoming ever-more fractured in its theoretical foundations, with any number of baseless hypotheses being accepted as fact, and passed off as legitimate theory to unwitting beginners.
But in fact there are beginnings of such a mathematical description, partially described right here on this blog. The elements are described by explicit geometric properties like extension (extroversion) and limitation (introversion). A distinct hierarchy emerges: the irrational elements are more fundamental and "wider" ontologically than the rational elements. Irrationality deals with direct apprehension, both physical (the senses) and mental (the imagination and memory). In other words irrationality is prelinguistic while rationality is linguistic. But rationality can be seen as the culmination of the system (and the intellect it describes), and perhaps gives it a greater degree of closure. All of the IM elements are good without a doubt in their own way, but they do play different roles and some are preferred over others in particular contexts.
As mentioned in another article, the contrary elements (Ne and Ni, Se and Si, etc.) are in fact the same information but opposite "vectors" or preferences within it. The dual elements are different, seemingly alien perspectives on the same reality, and our duals open us up to this perspective. The superego elements are arguably the "most opposite" from a higher perspective, but they share certain deceptive similarities and are "far apart" enough that contrary elements will conflict more readily. Conflicting elements can be reconciled in the long-term view but they certainly conflict in the here-and-now, as opposite states or choices (which produce said states).
That's roughly how it goes for IM elements. What about types? One benefit of 16-function models is that they identify types with IM elements, and therefore also functions with relationships. "Introverted socionics" further identifies types with relationships. Taken together, they will theoretically unite the basic elements of socionics into one fundamental reality, which manifests itself in different forms. It just so happens that IM elements are at the forefront of what we can describe in detail, because they are what we actually observe (i.e., information). So if you want to understand socionics, understand the IM elements.